The invention relates to a method and a device for the photothermal inspection of a material.
Photothermal inspection methods and devices are known which make it possible for example to effect the non-destructive testing of parts, so as to detect defects, variations in the nature or properties of the material of the part or differences in thickness of a coating layer of the part. These methods and devices may also be used to characterize local variations in diffusivity or in thermal conductivity at the surface or beneath the surface of a part made of the material. The part under inspection can be metallic and consist of a ferrous material, for example an alloy steel such as a stainless steel, or else of a non-ferrous material. The material may also be a composite, a ceramic material or a plastic.
The method of photothermal inspection which is carried out on a part or sample of material to be inspected uses the phenomenon of diffusion of a thermal disturbance produced by local warming of the part or sample. Investigation and characterization of the diffusion of the heat and of its variations at the surface of the part make it possible to detect or to characterize the part by detecting local variations in thermal diffusion.
Customarily, the device used or photothermal camera comprises a laser which is focused onto the surface of the part or sample under inspection, in a heating zone. The infrared flux radiated by the part in a detection zone neighboring the heating zone makes it possible to measure or evaluate the rise in temperature in the detection zone, due to the heating in the heating zone. The shift between the heating zone and the detection zone is generally referred to as the xe2x80x9coffsetxe2x80x9d. The flux radiated or the rise in temperature can be measured without contact by using a detector such as an infrared detector. The thermal flux radiated or the rise in temperature in the detection zone is influenced by the local characteristics of the materials examined. In particular, the diffusion of the heat between the heating zone and the detection zone which is the cause of the rise in temperature in the detection zone depends on the defects in the material, such as cracks, in the vicinity of the heating zone or of the detection zone or in the vicinity of both these zones.
In order to carry out non-destructive testing of the part made of the material or to carry out measurements of thermal diffusion in the material, a surface of the part or sample is scanned by displacing, at the surface of the part or sample, a means of imparting heat which generally consists of a laser beam focused onto a portion of the surface of the part constituting a heating zone.
The rise in temperature or the thermal flux radiated are determined in a detection zone whose position varies with the position of the heating zone, during the scan, the heating and detection zones being separated by a distance constituting the offset.
It is thus possible to obtain a two-dimensional image of the surface of the sample which is representative of the variations in diffusion of heat in the sample or else of the defects present inside the sample. Scanning can be achieved by using a fixed part and by displacing, over the surface of the part, the means for imparting heat synchronously with the detection zone; generally, scanning is carried out by deflecting a laser beam focused onto the surface, by using motorized steerable mirrors which are also used to send the flux radiated by the detection zone to a detector for measuring rise in temperature such as an infrared detector.
It is also possible to displace the part under examination past a fixed heating and detection device.
The scanning of the surface of the part is generally carried out by displacing, line by line over the surface, a pair of pointwise heating and detection zones, the heating and detection zones having for example circular shapes and small dimensions. As indicated above, a substantially point-like heating zone can be obtained by focusing a laser beam by means of spherical lenses onto the surface of the part or sample. Likewise, the infrared radiation emitted in a detection zone of small dimensions neighboring the heating zone is transmitted to an infrared monodetector in the form of a beam which is focused onto the sensitive surface of the detector which is generally of rectangular shape and of small dimensions.
The shift between the heating zone and the detection zone, referred to as the xe2x80x9coffsetxe2x80x9d is altered by fine mechanical adjustment of the position of the infrared detector or of its focusing optics, in a plane parallel to the surface of the part under inspection.
Such a method or device using point-like heating and detection zones has certain drawbacks.
This method and this device may be fairly easily adapted to materials which are poor conductors from the thermal point of view and highly absorbent or emissive from an optical point of view. Specifically, the laser sources implemented in the case of active thermography have powers varying from a milliwatt to a few watts. These powers are insufficient in the case of materials which do not exhibit the abovementioned characteristics and in particular in the case of most metals.
Furthermore, these methods and devices have a low measurement productivity since it is necessary to perform a complete scan of the surface of the part or sample by displacing the point zones of heating and of detection.
The surface portion corresponding to the detection zone and the scanning rates to be implemented to obtain measurable signals are generally too small for the measurements performed to exhibit a genuine economical benefit, in particular in the case of the inspecting of metals and in the case of any material in which the diffusivity gradients which one wishes to determine are small, for example because they are due to small-sized defects.
The adjusting of the offset between the heating and detection zones which must be achieved by fine positioning and fine adjustment of the optical elements for heating and for detection or of the detector may be lengthy and tricky to implement. This adjustment must be reviewed periodically when it is desired to obtain good reproducibility of the inspection performance. In particular, it is in practice necessary to work at constant offset, thereby ruling out any possibility of optimizing the signal recorded and to work at zero offset if it is desired to carry out double scanning of the part so as to eliminate the effect of the surface condition of this part; the scan must also be limited to a single direction because the offset must be parallel to the direction of displacement of the heating zone and of the detection zone.
The inspection device can also have considerable proportions on account of the dimension of the laser source required, thereby ruling out the use of devices for testing parts on site, when the surfaces of these parts are difficult to access.
The optical heating and detection systems customarily used make it possible to obtain xe2x80x9cfield depthsxe2x80x9d of a few millimeters at best, these xe2x80x9cfield depthsxe2x80x9d consisting of intervals of variation of the distance between the inspection device and the surface of the part, in which intervals it is possible to obtain guaranteed performance of the inspection device. This low depth of field makes it impossible to examine parts which do not have strictly plane surfaces, this being the case for most of the parts to which thermographic testing may be applied.
It has been proposed, in the case of a thermographic inspection, to use several detectors operating in parallel, possibly in the form of an array (WO 8700632). However, this use of a set of detectors does not by itself make it possible to eliminate the drawbacks recalled above of the process for the thermographic inspection of a material.
The purpose of the invention is therefore to propose a method for the photothermal inspection of a material, consisting in carrying out the heating of a heating zone at the surface of a part made of the material, using a means of imparting heat, the detecting of a flux radiated by the surface of the part in a detection zone some distance from the heating zone, and the relative displacing of the heating zone and of the detection zone at the surface of the part along a defined scanning path, this method making it possible to avoid the drawbacks of the prior art methods which were described above.
For this purpose, the detecting of the flux radiated by the detection zone whose position varies with the position of the heating zone is carried out by selecting a group of detectors from among a set of detectors, the detectors of the group of detectors being arranged in such a way as to receive a flux radiated by the detection zone of the surface of the part, so as to optimize the photothermal inspection and to increase the speed of execution of this inspection.